Animal rights extremists have threatened attacks on nearly 100 companies which donated money to Oxford University. The Animal Liberation Front (ALF) gave them a deadline of a week before attacks start on the homes of directors and employees, unless they promise never to give money to the university again.

They made the threat because a medical research laboratory at the university which will do animal-testing is being built. Work resumed on it in December, after a 16-month postponement due to intimidation and threats of violence.

University insiders spoke of a "climate of fear" on campus, students have been warned to step up security, and research scientists said they were bracing themselves for "very nasty activity by the ALF".

The group threatened to "trash" the offices of companies which refused to pull out and to attack the homes of directors, trustees and employees. Home addresses would be posted to other activists on the internet. "Any company who has not made an announcement can now expect full attention from the Animal Liberation Front," it said. "It's not going to be pretty."

Police are liaising with the FBI, who warned there is a "nexus" of animal rights extremists from both sides of the Atlantic working together. A spokesman for the FBI said there are "worrying signs" that violent action is on the increase.

The ALF has not officially stated who its targets are but is believed to have used a list of companies posted on the website of Speak, a group which is leading the campaign against the laboratory. Many of the firms are large multinationals, including IBM, Vodafone and BP, but there are also several environmental organisations such as the British Deer Society, the Galapagos Conservation Trust and the People's Trust for Endangered Species.

An insider at one threatened company said even if no attacks followed, damage would still be caused. "Most large corporate companies are averse to seeing their name associated with anything negative. The ALF hope if they spread the net wide enough, someone will blink, then they can say, 'Look, so-and-so company has pulled out.'"

But police believe violence, criminal damage and arson are likely. Top-level executives at the companies have met and taken advice from executives at Huntingdon Life Sciences (HLS), the animal research company that has been the focus of animal rights campaigners for the past decade.

Officers from Thames Valley Police are visiting the homes and offices of those targeted to advise on security. They are liaising with colleagues in Cambridgeshire who have dealt with animal rights extremists at HLS. Police believe the same people are behind threats at both sites.

The violence at HLS has refused to subside. In the latest attack, the home of a senior executive at a firm linked to HLS was covered with animal rights slogans and paint-stripper was poured over his car.

The ALF's deadline to Oxford University passed last week. But Robin Webb, the organisation's spokesman, said attacks were unlikely soon. "It will be a longer-term thing, as and when the opportunity arises. It will probably be over several months."

The campaign against the lab has been stepped up since work restarted. The building site is surrounded by a 15ft barrier, topped with two rows of razor wire. Builders wear balaclavas to avoid being identified. The construction company has not been named. The university has obtained an injunction limiting protests opposite the site. Activists are allowed to protest for three hours on Thursday afternoons and hold monthly demonstrations in the city centre. A spokesperson for Speak insists they are peaceful and have no connections with the ALF.

About 25 campaigners joined the protest last Thursday, under the eyes of more than a dozen police officers. Many were prepared to condemn those who resort to violence, but still employed the caveat that they "understand their frustrations".

Aside from their ethical objections, activists claim that testing on animals is not an effective way of finding medical cures. But Dr Simon Festing, the director of the Research Defence Society, representing 5,000 medical researchers, said: "More than 70 per cent of Nobel prizes for medicine have been based on animal research.

"Almost every major advance, from insulin for diabetes to the polio vaccine and heart transplant, has been based on animal research. Animal research has saved millions of lives."

FIVE YEARS OF FEAR

FEBRUARY 2001

Brian Cass, managing director of Huntingdon Life Sciences, is attacked by three men brandishing pickaxe handles outside his home in Cambridgeshire. He is left with a three-inch head wound. One man is jailed for three years.

2003-04

A campaign of intimidation and violent attacks is waged against the village of Newchurch in Staffordshire after the owners of a farm breeding guinea pigs for medical research refuses to close. More than 450 separate incidents are reported, including attacks on the local pub and threats to villagers.

JULY 2004

Oxford University halts the construction of the medical research laboratory after construction firm Montpellier pulls out. Threatening letters are sent to the company's shareholders and the value of its shares plummets.

OCTOBER 2004

The campaign against the guinea pig farm intensifies. The body of Gladys Hammond - mother-in-law of the owner, Chris Hall - is disinterred and stolen in Yoxall, Staffordshire.

OCTOBER 2005

A group calling itself the Animal Rights Militia sends death threats to companies working for Huntingdon Life Sciences. It also targets Leapfrog Day Nurseries for offering vouchers to workers at HLS.